The bipolar junction transistor (BJT) is the most important semiconductor device. This kind of device can be used in a high-speed logic circuit as a high power device. However, this device consumes a lot of power during operation. Therefore, the BJT has been replaced with the MOSFET in high-speed logic circuits to conserve power.
Generally, operation of the power-MOSFET is same as the operation of a typical MOSFET. However, the power-MOSFET can accept the larger current than a typical MOSFET. The voltage between the source electrode and the drain electrode of a power-MOSFET is about 20 to 1200 volts. The input impedance in the gate electrode of a power-MOSFET is large. Therefore, when a control voltage is applied to the gate electrode, the gate current can be kept low. In other words, it is only necessary to use a small control voltage to switch the power-MOSFET.
Reference is made to FIG. 1, a cross-sectional view of a conventional power-MOSFET. An N-type epi-layer 112 is deposited over a heavily doped N-type (or P-type) semiconductor substrate 100. Then, a poly-silicon gate layer 110 is defined over the N-type epi-layer 112. Two lightly doped P-well regions 108 are formed in the N-type epi-layer 112 as the channel region of the power-MOSFET. Two heavily doped N-well regions 114 are respectively formed in the two lightly doped P-well regions 108 as the source regions of the power-MOSFET. Finally, two metal layers 104 are respectively formed over the two heavily doped N-well regions 114 as the contact regions 102 and 104 with the source regions.
In accordance with the device depicted in the FIG. 1, the heavily doped N-well region 114 is the source region. The lightly doped P-well region 108 is the channel region. The heavily doped N-type semiconductor substrate 100 is the drain region. During operation, the electrons start from the source region and transversely pass through the inverse layer under the gate electrode to reach the N-type epi-layer 112. Then, the electrons can vertically pass through the he N-type epi-layer 112 to reach the drain electrode 100. Therefore, a wider channel region can raise the operation speed of a power-MOSFET. However, the wider channel region also increases the device size.
On the other hand, the conventional device structure is easily affected by a shift of the photolithography process, even if the shift is very small. Reducing the influence of the photolithography process is therefore highly desirable.